The Martensitic transformation behavior and microstructural features in irradiated NiTi shape memory alloy

In the present study, effect of He⁺ irradiation on the microstructural features and martensitic transformation of NiTi shape memory alloy was investigated. The results revealed that He+ irradiation induced the generation of irradiation defects such as He bubbles and dislocations. With the increased He⁺ dose, the He bubbles gradually grow up and merge. In addition, multiple-layer structure consisting of Ti₂Ni layer, B2 austenite phase layer and B19′ martensite phase layer can be observed in NiTi shape memory alloy irradiated with higher dose of 1.0 × 10¹⁷ ions/cm², which can be attributed to the variation of chemical composition induced by inverse Kirkendall effect and the preferred sputtering effect. In proportion, As the He⁺ irradiation dose increased from 0 to 5.0 × 10¹⁶ ions/cm², only one-stage B19′↔B2 martensitic transformation can be detected in NiTi shape memory alloy. Moreover, the martensitic transformation temperature is reduced with increased He⁺ dose. Upon He⁺ dose is increased to 1.0 × 10¹⁷ ions/cm², NiTi shape memory alloy exhibited two-stage martensitic transformation corresponding to B19′↔B2 martensitic transformation, which is due to the significant distinction of chemical composition between the outermost irradiated layer and unirradiated matrix layer.